This study is designed to delineate the relationship between the structural domains of factor IX and factor X and their overall enzymatic function. Factor IX and factor X are highly homologous but have a big disparity in function. Even though factor IX and factor X have a high degree of homology there are several regions of vast disparity in amino acid sequence. Study of these regions of difference between factor IX and factor X will help pinpoint regions of these molecules specific for particular functions such as interaction with cofactors, binding to phospholipid surfaces. interactions with substrates, and catalytic activity. This information could lead to the development of chromogenic substrates for factor IX allowing easier testing of activity and further our understanding of the clinical defect in patients with bleeding disorders. The mutant proteins will be produced by sit-directed mutagenesis. The regions of interest for mutagenesis include the gamma carboxyglutamic acid domain and particular regions of the heavy chain which appear to have an impact on the catalytic domain. The mutant proteins will be characterized in comparison to plasma derived and recombinant wild type protein. The characterization studies will include calcium ion binding, binding to phospholipid surfaces, interaction with cofactors (FVIIIa and FVa), and inhibition by antithrombin III.